1. Field of the Invention
The invention relates to a method for compensating for magnetic noise fields in spatial volumes.
2. Description of the Related Art
In the following description and claims, “noise fields” shall be intended as magnetic field fluctuations induced in a spatial volume, particularly the imaging cavity of Magnetic Resonance imaging apparatuses, which fluctuations are caused by magnetic fields outside said spatial volume or the imaging cavity of the MRI apparatus.
Currently, the wide use of electric power causes the generation of considerable magnetic fields, which pervade the environment. These magnetic fields may have considerable strengths and affect or alter the operation of electric or electronic equipment which use magnetic fields. Such equipment may be of any type, e.g., measuring instruments, diagnostic or therapeutic imaging apparatuses, and the like.
Currently, the above noise may be caused by different types of magnetic fields, which are differentiated on the basis of frequency. A first so-called low-frequency type includes magnetic fields with frequencies ranging from less than 1 Hz to a few units of Hz. These types of low-frequency noise fields are typically generated by the passage of vehicles or the like. Each vehicle generates, in first approximation, a magnetic dipole with a predetermined strength and a predetermined position and the dipoles are oriented in the direction of the earth's magnetic field flow lines.
A second type includes noise fields generated by sources of mains AC, which have frequencies of about 50 to 60 Hz. Besides home or static sources, electric vehicles shall be also considered, such as trains, tramcars, subway trains, trolley-buses, etc., due to the considerable powers absorbed and the strengths of the fields generated thereby.
A third category might include noise fields with frequencies of the order of a fraction of the main AC frequency, i.e., of about 10 to 20 Hz. Particularly, some railway electrification lines use, for instance, one-third of the main AC frequency, that is a frequency of the order of 16 Hz.
Other noise fields consist of fast, i.e., high-frequency transients. In spatial volumes containing significant electrically conductive masses, e.g., shielding magnetic structures or Faraday cages, these fast transients induce noise currents, which modify the noise fields inside the imaging cavity. These fast magnetic field transients may be also combined with other noise types.
Currently, the methods of compensation for magnetic fields like the ones described hereinbefore use a single sensor and jointly compensate the fields with different frequencies. Joint processing extends from the detection of noise fields, through the generation of compensation fields, to the calculation of the noise fields generated inside the spatial volume, based on the results of measurements outside the spatial volume.